There are many techniques in the prior art for milling material from a workpiece. One of the most important of these is lineal milling, or machining, which is characterized by a milling motion which utilizes milling forces parallel to the center line rotation of a spindle or cutting tool to remove metal from the workpiece. This is to be distinguished from conventional milling which typically advances a rotating cutting tool perpendicular to the center line of rotation into a workpiece. Lineal milling provides certain advantages over conventional milling. First of all, the milling forces are supported by the compression strength of the spindle and cutting tool and the spindle thrust bearings as opposed to the lateral rigidity of the cutting tool and spindle which is typically much weaker. Furthermore, greater depth in the opening may be achieved more easily with lineal milling as the length of the cutting tool may simply be increased while increasing the length of the cutting tool for conventional milling further decreases the lateral strength of the tool which can result in deflection of the tool and a failure to achieve the desired result. Therefore, for these and other reasons, lineal milling is much more desirable in many applications than conventional milling.
Perhaps the simplest form of lineal milling is exemplified by a drill press which utilizes a conventional drill bit which may be advanced into a workpiece. However, a drill press is unsuitable for drilling pocket-type openings in that advancing the drill bit into a flat surface is relatively easy while advancing the drill bit into a workpiece such that it overlaps a previously drilled hole has a tendency to deflect the bit. Thus, additional steps with other equipment is required to finish off the machined opening.
The inventor herein is aware of at least one attempt in the prior art to decrease the tendency of the drill bit to deflect as the bit is advanced into a workpiece. In this prior art device, a spindle is used to mount the drill bit to the machine, and the spindle has a female taper, or inner taper, and the drill bit has a matching male taper such that it slides within the opening or envelope provided by the spindle and is secured thereto with a spindle drawbar or the like. While this does improve the support for the cutting tool as it increases the width or base for support of the tool, there are difficulties in implementing it.
First of all, truing of a female taper on a spindle can be quite difficult and time-consuming (and thus expensive). Furthermore, requalifying of the inside taper or female taper of a spindle, as might be expected after some use, would require at least partial disassembly of the spindle.
In order to solve these and other problems in the prior art, the inventor herein has developed a lineal machining tool which can be used to great advantage in pocket-type machining applications, as well as other applications. The lineal machining tool and method of the present invention permits lineal machining of overlapping holes in a manner which "stomps" or "nibbles" material away from the workpiece with successive advances of the machine tool into the workpiece. The machine tool is characterized by a cutting tool which has a female taper which matches and lines up with the outer male taper of the spindle, both surfaces of which are machined or "trued" so as to accurately position the cutting tool with respect to the center line of the spindle. The cutting tool is mounted to the spindle much as with a standard cutting tool, i.e. with a spindle drawbar or center mounting bolt which urges the tapered surfaces into engagement. However, with this arrangement, for the first time, the cutting tool is supported at the widest dimension of the spindle to provide the greatest amount of lateral support to resist deflection of the cutting tool as it is advanced into the workpiece in a lineal machining or milling motion.
Still another feature of the cutting tool of the present invention is that a plurality of replaceable carbide cutting edges are mounted along the outer periphery of the outboard end of the tool such that, when the tool is rotated, the cutting edges circumscribe an annulus which comprises less than all of the surface area of the outboard face. Thus, the cutting tool isolates its cutting surface such that it is uniquely suited for being successively advanced in an overlapping manner into a workpiece. Thus, cutting edges are maximally utilized, especially in a CNC machine, where successive advances of the cutting tool may be accurately controlled to choose the amount of overlap to best advantage.
Because of the tremendous rates at which material is removed from the workpiece, flutes are formed in the body of the cutting tool such that as the cutting edges shave chips from the workpiece, they are directed by the cutting edges up through the flutes and away from the workpiece. Furthermore, lubrication ports are formed in the cutting tool from the center draw bolt to near the carbide cutting edge so that air or lubricating oil may be delivered directly to the cutting edge and surface of the workpiece.
As already alluded to above, the inventor has also succeeded in developing a method for lineal milling which utilizes the tool of the present invention to great advantage. That method includes advancing the cutting tool into the workpiece such that the cutting annulus circumscribed by the cutting edges at least partially overlap the cutting annulus of the previous pass by the cutting tool through the workpiece. After each cutting stroke, the tool is retracted from the workpiece, indexed to a new overlapping position, and then advanced back into the workpiece. With this motion, the tool can be used to great advantage to "nibble" or "stomp" material from the workpiece at tremendous rates in a CNC machine through the use of simple programming techniques as is well known in the art.
While the principal advantages and features of the invention have been described above, a clearer understanding of the invention and its purposes may be attained by referring to the drawings and description of the preferred embodiment which follow.